For modern systems of illumination, whether illumination associated with ambient lighting, image capture, image projection, image viewing, signage illumination and/or projection, etc. it is often desirable to generate one or more specific narrow spectra of light. In particular, it is often desirable to generate narrow spectrum light corresponding to one or more of the additive primary-color components red (“R”), green (“G”) and blue (“B”) and/or spectra corresponding to one or more of the subtractive color components magenta (“M”), cyan (“C”) and yellow (“Y”). A full set of such primary color narrow spectra may be color-balanced to create white light. White light may in turn be filtered to create any other color.
Another example use of a set of primary colors of light is the time-sequencing of each primary color onto a digital micro-mirror device (“DMD”) associated with a Texas Instruments Incorporated Digital Light Processing (“DLP”)™ projection system. Each micro-mirror of a two-dimensional matrix of micro-mirrors on the DMD surface may be separately re-positioned at the start of each primary color time slot to reflect a single pixel of the current primary color into or away from an optical projection system. Doing so produces a projected two-tone pixel image of the current primary color. A series of such two-tone pixel images projected while rapidly sequencing between primary colors is integrated by the human eye to create the illusion of full-color image frames seen as a still or moving picture.
Many systems of illumination require significant light power expressed in lumens. In turn, energy efficiency standards often dictate minimum light generation efficiency expressed in lumens of output light power per watt of electrical input power. A laser is a potentially powerful and efficient light source due to its low etendue and narrow spectral band. In particular, blue light emitted at approximately 448 nm and ultraviolet (“UV”) light emitted at approximately 405-420 nm are energetic and can be generated by lasers at high efficiencies. To take advantage of this phenomenon and to engineer simpler multi-wavelength illumination systems, light from one or more blue or UV lasers may be used to excite one or more luminescent phosphors coating one or more portions of a surface of a wavelength conversion element such as a phosphor wheel. Various phosphors are available, each capable of luminescing in a narrow spectrum when excited by a particular excitation spectrum of light. This technique may be used to create multiple primary colors from a low etendue, narrow band excitation light source.
FIG. 1 is a prior-art schematic diagram of a multi-wavelength light generation apparatus 100 using a low etendue, narrow band excitation light source 110. The excitation light source 110 is an emitter of high-energy light 115 (e.g., a blue laser). The emitted light 115 is processed by one or more condensing, collimation, diffusion and/or beam shaping excitation optical element group(s) 118 to generate an excitation beam 122. The element group 118 may include a beam-shaping element such as an optical diffuser, for example. The beam-shaping element homogenizes the excitation beam 115 from the excitation light source 110 in order to better distribute the intensity of the excitation beam 115 over the beam area at the phosphor wheel 138. The excitation beam 122 is reflected by a dichroic mirror 127 through condensing and collimation optics group 133 and onto a surface of a phosphor wheel 138. Each of various phosphors coated onto various areas of the surface of the phosphor wheel 138 luminesces at a predetermined wavelength as it is illuminated by the excitation beam 122.
It is noted that the various phosphor-coated areas may be exposed to the excitation beam 122 at different times by locating the areas radially around the wheel surface and rotating the wheel. Doing so may desirably temporally separate the output colors. Each resulting wavelength of light emanating from the phosphor wheel 138 (e.g., as represented by light rays 143, 148, and 150) corresponds to a desired output color (e.g., R 153, G 158 and Y 160). The phosphor-emitted wavelengths are collected and collimated by the optics group 133 and are passed to the output 165 through the dichroic mirror 127.
If the excitation light source 110 emits light of a visible wavelength such as blue as illustrated in the example apparatus 100, it may be desirable to include the excitation light color in the color sequence at the output 165. However, doing so is not easily accomplished by simply reflecting light of the excitation spectrum from the phosphor wheel surface, because the dichroic mirror is designed to reflect and not pass light of the excitation spectrum. Consequently, such a dichroic mirror-based system may include a separate light source 170 to emit light of a color corresponding to the excitation spectrum for sequencing at the output 165. The latter configuration may also include one or more condensing, collimation, diffusion and/or beam shaping optical element group(s) 175. Such additional components add cost and complexity.
FIG. 2 is a prior-art schematic diagram of a multi-wavelength light generation apparatus 200 using a low etendue, narrow band excitation light source. The apparatus 200 includes one or more blue light excitation lasers 110, light 115, excitation light optical element group 118, excitation beam 122, dichroic mirror 127, condensing and collimation optics group 133, phosphor wheel 138 and output 165, all as described above with reference to FIG. 1. Additionally, the apparatus 200 includes an opening 210 in the phosphor wheel (e.g., a slot along a radius) to pass light of the excitation spectrum (e.g., blue) at a time when the color corresponding to the excitation spectrum is desired. A series of mirrors (e.g., the mirrors 215, 220 and 225) create a “wrap-around” path 230 to direct light in the excitation spectrum to a diffusion and collimation optical group 235. Collimated light in the excitation spectrum is subsequently reflected by the dichroic mirror 127 to the output 165. The wrap-around path 230 may increase the overall size of the apparatus 200.
As noted, the prior-art apparatus 100 and 200 may both include a duplication of optical elements such as diffusers and/or beam-shaping elements. One set of duplicate elements is included in the excitation light optics group 118 to process the excitation light 115. The other set of duplicate optical elements is included in the element group 175 of the apparatus 100 and the element group 235 of the apparatus 200 to process blue light to be sequenced at the output 165. The duplicate optical elements add cost and complexity.